In telecommunications systems, the quality of the received signals is affected by quick, impulsive interference due to various factors, such as multipath propagation, fading of transmitted signals, shadowing, near-far problem and co-channel interference. The impulses disturb the signal-processing methods used in telecommunications systems, due to which estimating the power of a received signal, for instance, may be unsuccessful.
Power estimation is used in radio systems, for instance in estimating the signal-noise ratio and setting threshold values as well as in automatic amplification control. However, the impulses disturb classical power estimation algorithms and can lead to erroneous results. Cancelling impulsive interference is often a prerequisite for the receivers to be able to operate at a sufficient accuracy. For instance in FFT (Fast Fourier Transformation) band-stop filters used in receivers it is important to find the suitable threshold value level, because the performance of the band-stop filter depends to a great extent on the correct threshold level setting. Further, when signal and noise subspaces are separated, different noise-attenuation methods are used in which subspaces are separated on the basis of information-theoretical criteria, such as Akaike and MDL.
It has become more and more common to use so-called robust, i.e. control-weighted methods for cancelling impulsive interference in the telecommunications systems. The robust methods are not sensitive to big changes in individual observation values, such as impulse-like interference in a received signal. The prior art robust methods utilize so-called order statistics, the basic idea of which is to detect and cancel observed interference by properties associated with an observation set arranged on the basis of variable values. One prior art robust method is so-called median-type filtering. This kind of prior art median-type filtering, used for power estimation, is described in greater detail for instance in the publication by C. Tepedelenlio{hacek over (g)}lu, N. Sidiropoulos, G. B. Giannakis, “Median Filtering For Power Estimation In Mobile Communications Systems”, Third IEEE Signal Processing Workshop on Signal Processing Advances in Wireless Communications, Taoyuan, Taiwan, Mar. 20-23, 2001, pp 229-231.
A drawback of the prior art solutions is, however, that the ability to endure impulsive interference is not sufficiently good. Thus, they are not well suited for separating signal and noise subspaces, for instance. In a median-type method according to the prior art, a drawback is that information on the interfering signal is needed in advance to be able to identify the correct interference impulses. Furthermore, it is difficult to implement a median-type method in practice.